1. Field of the Invention
The present invention relates generally to a method and apparatus for reading multilevel signals from an optical disc and writing multilevel signals to an optical disc. The invention relates to methods and apparatuses for processing signals that are eventually written to and read from an optical disc. These signals produce marks on the optical disc that may vary in both reflectivity and length. The system disclosed provides a method of encoding and decoding the data, correcting for errors, synchronizing the data, controlling the DC content, establishing and recovering a clock signal, establishing and recovering the envelope of the signal, and compensating for signal distortion.
2. Relationship to the Art
In order to increase the capacity and speed of optical data storage systems, multilevel optical recording systems have been developed. It should be noted that in this specification, the term multilevel is used to indicate greater than 2 levels. In a traditional optical recording system, reflectivity of the recording media is modulated between two states. The density of data recorded on an optical recording medium may be increased by modulating the reflectivity of the optical recording medium into more than two states.
One type of optical recording medium that appears to be particularly suitable for multilevel signal modulation is phase change optical material. When a phase change material is heated by a writing laser, the reflectivity of the phase change material may be changed. The change in reflectivity may be controlled by adjusting the amount of heating of the material and the rate at which the material cools. This process is described further in "Laser-induced crystallization phenomena in GeTe-based alloys. I. Characterization on nucleation and growth" (J. Appl. Phys. 78 (8), Oct. 15, 1995. p. 4906) by J. H. Coombs, et. al. (hereinafter "Coombs").
After a phase change optical disc has been written, the intensity of a beam of light reflected from the disc is measured so that the multilevel data written to the disc may be recovered. U.S. Pat. No. 5,144,615 entitled APPARATUS AND METHOD FOR RECORDING AND REPRODUCING MULTILEVEL INFORMATION issued to Kobayashi (hereinafter "Kobayashi") discloses a system for recovering multilevel data from such an optical disc. FIG. 1 is a block diagram illustrating the system disclosed in Kobayashi for recovering such data. Analog data read from a detector is input from a mark length detecting circuit 101 and a reflectivity detecting circuit 102. The outputs of these circuits are sent to an analog-to-digital (A/D) converter 103. The A/D converter 103 includes an n-value circuit which determines the value that the signal corresponds to by comparing the signal to predetermined reference voltages. Subsequently, the n-value signal is converted into a binary signal by binary circuit 405.
While this system discloses the concept of reading a multilevel signal and converting it into a digital signal in a basic sense, no method is disclosed of handling various imperfections in optically read multilevel signals that in fact tend to occur. For example, it is not clear how a clock is recovered for the purpose of precisely detecting mark lengths and no method is disclosed for handling problems that tend to occur in real systems such as amplitude modulation and DC offset of the optically detected signal and noise.
In a conventional two level optical data storage system, information is stored in the lengths of the marks and the spaces between them. So long as the edge of a mark can be detected with enough precision to distinguish between marks that differ in length by a minimum allowed amount, the system can operate reliably. This edge transition between one reflectivity state and another can be detected by setting a threshold value and determining the time when the signal crosses the threshold. Slow amplitude variations that might interfere with this edge detection are removed by AC coupling the photodetector signal before the threshold detection circuit. Mark and space lengths are measured by counting how many clock periods are between the edge transitions. The reader clock periods are synchronized to the mark/space edges, thus ensuring that there are an integral number of clock periods in each mark/space.
In contrast, in a multilevel recording system, it is the amplitude of the signal that carries information. The reader must interpret the data signal to determine the amplitude of the signal at certain times. Therefore, the reader clock must be synchronized to the data stream to ensure that the reader is interpreting the signal at the proper time. Because of the blurring effect of the optics in a reader, the transitions between the different levels do not create sharp edges. It is therefore difficult to synchronize the reader clock to the data stream. A method of precisely aligning a read data stream is needed. Further, a multilevel system is more sensitive to fluctuations in the overall envelope of the data signal. AC coupling alone is not adequate to enable a sufficiently precise determination of the different amplitude signals. Another problem encountered in a multilevel optical disc system is DC compensation.
In order for a multilevel optical read system to reliably record and recover data, a method of handling these sources of error in reading an optical signal is needed.